The invention relates to a brake unit for a road vehicle with a hydraulic dual-circuit braking system comprising a static front-axle brake circuit and a static rear-axle brake circuit which are connected respectively to a primary outlet-pressure space and a secondary outlet-pressure space of the brake unit. Brake pressure can be built up in these spaces under the control of an actuating force K.sub.P, set a a value correlated with the actuating force with which the driver actuates a brake pedal. The brake unit includes a hydraulic brake booster comprising a brake valve generating an outlet pressure which is proportional to the pedal force K.sub.P and which can be fed into at least one drive pressure space which is limited in an axially movable manner by a drive piston of the brake booster. The displacement of the piston occurs as a result of being subjected to pressure determined by the brake valve and this displacement can be transmitted, via at least one axial-force transmission element, to piston elements of a primary-piston arrangement forming one movable side of the primary outlet-pressure space. By this arrangement the primary-piston directly experiences the movement of the drive piston and a second piston limiting a secondary outlet-pressure space is moved indirectly as a result of the pressure build-up in the primary outlet-pressure space and into a position linked to the expected brake-force value. The brake system includes a housing possessing two housing bores, parallel to one another, in one of which the secondary piston is arranged in a displaceably pressure-tight manner and in the other of which, at least one piston element of the primary-piston arrangement is guided displaceably in a pressure tight manner. The effective cross-sectional surface of the said piston element defines a corresponding part cross-section of the primary outlet-pressure space. The primary-piston arrangement comprises a piston element which, in the event of a failure of the brake booster, remains in a basic position corresponding to the non-actuated state of the brake unit and which is motionally coupled to the brake pedal positively and non-positively and, in the event of a failure of the front-axle brake circuit, can be supported axially on a piston element, as a result of the axial displacement of which, brake pressure can be built-up in the secondary outlet pressure space.
A brake unit of this general type is known from German Patent Specification No. 3,632,507. This known brake unit is intended for a braking system with a static front-axle brake circuit and a static rear-axle brake circuit which are connected respectively to a primary outlet-pressure space and a secondary outlet-pressure space of the brake unit. Brake pressure can be built up in these outlet-pressure spaces under the control of the actuating force K.sub.P with which the driver actuates a brake pedal. Integrated in this brake unit is a hydraulic brake booster comprising a brake valve supplying an outlet pressure which is proportional to the force K, with which the driver actuates the brake pedal, and which is derived from an auxiliary pressure source. This outlet pressure, proportional to the pedal force, can be fed into a drive pressure space limited movably by a drive piston which is designed as an annular piston and which, as a result of the drive pressure space being subjected to pressure, experiences a displacement which takes place via a flange-shaped axial-force transmission element of a plunger piston passing axially through the annular piston. The plunger piston is sealed off and displaceable relative to the annular piston and forms, together with the annular piston, the primary-piston arrangement which forms the limitation, movable on one side, of the primary outlet-pressure space, to which the front-axle brake circuit is connected. The pressure build-up occurring in the primary outlet-pressure space causes the indirect displacement of the floating piston, delimiting the primary outlet-pressure space relative to a control-pressure space, in which a pressure corresponding to the pressure in the primary outlet-pressure space is built up as a result of the displacement of the floating piston. These pistons and pressure spaces are arranged along a common central longitudinal axis, along with a guided displaceable push rod, via which, in the event of a failure of the brake booster, the pedal force can be transmitted to the central plunger piston of the primary-piston arrangement. The effective cross-sectional surface of the plunger piston is smaller than that of the annular piston. The pressure which can be built up in the control-pressure space is fed, via a transverse channel in the housing of the known brake unit, into a second control-pressure space, limited fixedly relative to the housing by a housing bore, the central longitudinal axis of which extends parallel to the central longitudinal axis along which the above-mentioned pistons and pressure spaces are arranged. This second control-pressure space is limited, movably on one side, by a drive piston which itself engages on a tappet-shaped extension of the secondary piston, forming the movable limitation of the secondary outlet-pressure space to which the rear-axle brake circuit is connected. This "bent" design of the brake unit, with primary and secondary outlet-pressure spaces connected hydraulically in series, is chosen in order to make it possible to obtain a design arrangement having a short dimension in the axial direction.
The two-part construction of the primary-piston arrangement ensures that, in the event of a failure of the brake booster, a "ratio jump" occurs, so that, if the brake booster fails, that is to say when only the pedal force is available for actuating the brakes, a so-called "ratio jump" takes place. Hence, with a lengthened pedal travel, a relatively higher brake pressure can be built up. Furthermore, the floating piston, delimiting the primary outlet-pressure space relative to the first control-pressure space, is so designed that upon a failure of both the brake booster and the front-axle brake circuit, the plunger piston can come to bear on an axial extension of the floating piston, and therefore the floating piston is displaceable as a result of pedal force. Thus a brake pressure can thereby be built up in the rear-axle brake circuit, and because the floating piston has an effective cross-sectional surface smaller than the total cross-sectional surface of the primary-piston arrangement, a "pressure jump" takes effect, in such a way that the brake pressure, which can be built up in the rear-axle brake circuit during the particular malfunction mentioned, is relatively higher than in circumstances free of any malfunction.
Nevertheless, the known brake unit has the disadvantage, which must be considered serious from the point of view of safety, that when a leak occurs between the control-pressure spaces connected to one another by means of the transverse bore, it is no longer possible to actuate the rear-axle brake circuit. The same disadvantage occurs when the control-pressure spaces, connected to one another by means of the traverse channel, are poorly ventilated. In the design characteristic of the known brake unit, there is also fundamentally no possibility that the piston, movably limiting the secondary outlet-pressure space, can be actuated "mechanically" via the brake pedal, as a final possible emergency measure. The known brake unit is therefore less functionally reliable than a conventional tandem master cylinder, the disadvantage of which is to be seen in the fact that it requires a relatively large axial constructional length, which is often not available in the confined installation space.
The object of the invention is, therefore, to improve a brake unit of the type mentioned in the introduction, to the effect that, while having an axial constructional length reduced in comparison with a conventional tandem master cylinder, in emergency situations of a malfunction of the brake booster and/or a failure of the front-axle brake circuit, it offers the same degree of safety in terms of the brake forces or vehicle decelerations still obtainable under those circumstances as a conventional tandem master cylinder, yet is of simple construction and can be produced cost-effectively.
According to the invention, this object can be achieved by having the primary outlet-pressure space comprise a first part space which is movably limited on one side by a first piston element of the primary-piston arrangement and which is otherwise fixed in a first axial bore of the housing. A second part space is provided which is in constant communicating connection with the first part space and which is movably axially limited on both sides, within the second housing bore by the secondary piston on one side and on the other side by two piston elements of the primary-piston arrangement which are displaceable relative to one another in a pressure-tight manner. One of these two piston elements is designed as an annular piston, sealed off displaceably relative to the housing bore, and the second, piston is designed as a plunger piston which is arranged displaceably in a pressure-tight manner in an axial passage bore of the annular piston. A first yoke-shaped axial-force transmission element is provided to make a positive and non-positive motional coupling of the primary piston, limiting the first part space of the primary outlet-pressure space, with only one of the two piston elements which limit the second part space of the primary outlet-pressure space. A second axial-force transmission element is provided to make a positive and non positive motional coupling of the other piston element of the primary-piston elements, limiting the second part space, with the drive piston of the brake booster. A third axial-force transmission element is provided and via which, the actuating force, exertable by means of the brake pedal acts directly on the yoke-shaped axial-force transmission element in the event of a failure of the brake booster.
According to this, the primary outlet-pressure space comprises a first part space which is movably limited on one side by a piston element of the primary piston arrangement and otherwise fixed relative to the housing by the first axial bore of the housing. Furthermore, the primary outlet-pressure space comprises a second part space which is in constant communicating connection with the first part space and which is movably axially limited on both sides within the second housing bore. These axial limitations are formed, on the one hand, by the secondary piston and, on the other hand, by two piston elements of the primary-piston arrangement which are displaceable relative to one another in a pressure-tight manner. The primary piston arrangement has one piston designed as an annular piston, sealed off displaceably relative to the housing bore, and another second piston, designed as a plunger piston which is arranged displaceably in a pressure-tight manner in an axial passage bore of the annular piston. There is a first yoke-shaped axial-force transmission element which makes a positive and non-positive motional coupling of the primary piston, limiting the first part space of the primary outlet-pressure space, with only one of the two piston elements which movably limits the second part space of the primary outlet-pressure space on one side. Furthermore, there is a second axial-force transmission element, which makes a positive and non-positive motional coupling of the other piston element, or of the primary-piston elements limiting the second part space, with the drive piston of the brake booster. Finally there is a third axial-force transmission element, via which the actuating force, exertable by means of the brake pedal, acts directly on the yoke-shaped axial-force transmission element in the event of a failure of the brake booster. This design of the brake unit according to the invention guarantees that, in the event of a simultaneous failure of the brake booster and the front-axle brake circuit, the secondary piston limiting the secondary outlet-pressure space of the brake unit, to which the rear-axle brake circuit is connected, remains displaceable mechanically coupled to the brake pedal, and therefore the vehicle remains brakeable even in extreme emergency. In comparison with the known brake unit, this advantage is afforded without restriction of the requirement for as short an axial constructional length as possible. In constructive terms, the brake unit according to the invention involves a lower technical outlay than the known brake unit.
By means of a second drive piston, arranged coaxially relative to that piston element movably limiting the first space of the primary outlet-pressure space and which can be supported on the yoke-shaped axial-force transmission element, it is possible to achieve the higher boosting effect and, if appropriate, also the utilization of an auxiliary pressure source working at a lower outlet-pressure level. This is advantageous from the point of view of a reduction in wear and the functional reliability of the brake unit.
By a constructionally especially advantageous arrangement of the brake valve within this further drive piston, a push-rod-shaped control member of the brake valve, supported on the drive piston, can be utilized to introduce the actuating force, in the event of a failure of the brake booster, which actuating force is still available for actuating the brakes.
Depending on the design location of the booster pistons and of the brake-unit master-cylinder pistons, transverse forces occurring in the event of a failure of the brake booster or, in circumstances free of any malfunction and engaging especially on the yoke-shaped axial-force transmission element, which is exposed to asymmetric reaction forces at least in the event of a failure of the booster, can be absorbed sufficiently reliably in a simple way by means of an axial guide device. This is obtained by means of piston-like guide extensions, in mutually aligned guide bores of the brake-uint housing.
The design of the brake-unit housing wherein a block-shaped main cylindrical part and a likewise block-shaped booster part are formed at parting line which runs within a leakage-oil space in communicating connection with the outside atmosphere, provides a location for the yoke-shaped axial-force transmission element to be displaceable between positions corresponding to the non-actuated state of the brake unit and to the actuating state linked to the maximum brake force. This ensures a reliable media separation between the brake-unit outlet-pressure spaces, containing the brake fluid, and the brake-unit pressure spaces assigned to the booster part. This avoids the danger that quantities of hydraulic oil, by means of which the brake booster is operated, albeit even if only small quantities, can come in contact with the gaskets, by means of which the outlet-pressure spaces of a brake unit are sealed off from the leakage-oil space. In the brake unit according to the invention, therefore, it is possible to use the conventional gaskets which are resistant to only one hydraulic medium, namely brake fluid or hydraulic oil, and which are substantially cheaper than gaskets resistant to both media.
On the assumption of a uniform--good or bad degree of ventilation of the two brake circuits and fault-free functioning of the brake booster--the invention adheres to the dimensional relationship, that the ratio F.sub.4 /F.sub.5 of the cross-sectional surface F.sub.4 of the primary piston, movably limiting the first part space of the primary outlet-pressure space to the total cross-sectional surface F.sub.5 of the primary-piston elements movably limiting as a whole the second part space of the primary outlet-pressure space, is equal to the ratio F.sub.2 /F.sub.3 of the surfaces F.sub.2 and F.sub.3 of those drive pistons, which are supported axially on the respective primary piston or primary pistons via the axial-force transmission element. This provides for an effective cross-sectional surfaces of the primary pistons and of the drive pistons of the brake unit, which are arranged respectively coaxially with the primar pistons and thus ensures a perfect compensation of the drive and reaction forces engaging on the yoke-shaped axial-force transmission element, so that no transverse forces can act on the system.
This is true of the symmetrical design of the brake unit wherein, two housing bores are made in a block-shaped master-cylinder part in a symmetric arrangement relative to the central longitudinal axis of that part space of the primary outlet-pressure space limited only by one primary piston. Within these housing bores are two secondary outlet-pressure part spaces and two primary outlet-pressure part spaces, provided in a tandem arrangement. These bores house a floating piston and a primary-piston element in the form of an annular piston and a plunger piston. The secondary outlet-pressure part spaces are in communicating connection with one another and the two part spaces of the primary outlet-pressure space are connected to the part space limited by the centrally arranged piston. The axial actuating force controllable or exertable via the push rod, motionally coupled with the brake pedal, acts centrally, via the symmetrically designed yoke-shaped axial-force transmission element, on the primary piston limiting the central part space and, via radial supporting arms of the yoke on one of the two symmetrically arranged primary-piston elements.
In the design of the brake unit, the actuating force, controllable (or in the event of a failure of the brake boost, exertable) by means of the brake pedal, operates along the common central longitudinal axis of the housing, between the central longitudinal axes of the housing bores and parallel to these. Guide extensions of the yoke-shaped axial-force transmission element also are provided in these housing bores. Thus even with an asymmetric arrangement of the pistons, limiting the outlet-pressure spaces of the brake unit, a marked reduction of the transverse forces taking effect on the yoke-shaped axial-force transmission element in the event of a failure of the booster is achieved.
Both in combination with this design and in combination with a symmetrical design of the brake unit, it is possible and advantageous to provide a third drive piston which is arranged coaxial relative to the central axis, marking the direction of engagement of the actuating force exertable by the brake pedal, and which contains the brake valve of the brake booster and engages on the yoke shaped axial-force transmission element via the booster-facing guide extension of the latter. This arrangement can be utilized to increase the boosting effect or to lower the outlet-pressure level of the auxiliary pressure source of the brake booster. Alternatively the yoke-shaped axial-force transmission element can engage the plunger piston of the primary-piston arrangement via a radial supporting arm for limiting the second or third part space of the primary outlet-pressure space of the brake unit on one side. The plunger piston is connected fixedly in terms of displacement to the yoke-shaped axial-force transmission element and the annular piston can be supported directly on the drive piston of the brake booster via tappet-shaped axial-force transmission elements extending laterally past the supporting arm. Alternatively the yoke-shaped axial-force transmission element could engage on the annular piston of the primary-piston arrangement, limiting the second and/or third part space of the primary outlet-pressure space of the brake unit, via a radial supporting arm. There the plunger piston of this primary-piston arrangement is motionally and fixedly coupled in terms of displacement to the drive piston by passing axially through a bore of the supporting are of the yoke-shaped axial-force transmission element. The designs of the yoke-shaped axial-force transmission element offer possibilities for supporting it respectively on one of the two primary-piston elements, which jointly limit on one side, a part space of the primary outlet pressure space of the brake unit. Each of these two alternative designs of the brake unit ensures that in the event of a failure of both the brake booster and the front-axle brake circuit, a so-called pressure jump takes effect on the rear-axle brake circuit by pedal force. For an acceptable lengthening of the pedal travel, this pressure jump makes it possible to build up a relatively higher brake pressure in the rear-axle brake circuit in comparison with brake unit without a device of this type.
By having, in the basic position of the pistons and axial-force transmission elements of the brake unit (corresponding to the non-actuated state of the latter), a free end of the plunger piston located on the same side as the pressure space (by means of which the latter can be supported on the secondary piston in the event of a joint failure of the brake booster and of the front-axle brake circuit) arranged at a greater axial distance from that piston flange of the secondary piston facing the primary outlet-pressure space than an end extension of the annular piston pointing towards the said secondary piston, a shorter construction is possible. Alternatively by having, in the basic position of the pistons and axial-force transmission element of the brake unit (corresponding to the non-actuated state of the latter), the free end of the plunger piston, located on the same side as the pressure space (and by means of which the latter can be supported in on the secondary piston in the event of a failure of the front-axle brake circuit) arranged at a smaller axial distance from the secondary piston than an end extension or the annular piston (which points towards the said secondary piston and by means of which the latter can be supported on the secondary piston) also provides for an ideal construction in the event of a failure of the brake booster.
These two alternative designs of the brake unit (having the primary-piston element in the form of a plunger piston and of an annular piston) avoid, in the event of a failure of the brake booster in the partial braking range, a differing pressure generation in the front-axle brake circuit and in the rear-axle brake circuit. A differing pressure generation in the front-axle brake circuit and in the rear-axle brake circuit can occur only when the primary-piston element, "taken up" by the yoke-shaped transmission element, runs onto the secondary piston of the brake unit. The substantial avoidance of a pressure spread obtainable in this way is important in an emergency to prevent over-braking of the rear axle in the partial braking range. The advantage is that the vehicle is guaranteed good dynamic stability during emergency braking. On the other hand, if the front-axle brake circuit fails, this dimensioning of the primary pistons means that that piston located at a smaller distance from the secondary piston runs onto the secondary piston correspondingly "earlier", and this is also beneficial to the practical need for as early a start as possible of the braking effect on the rear axle. In this emergency situation, the brake unit according to the invention works in the same way as a conventional tandem master cylinder.
The installed brake-force distribution can be set to two different set values by means of a change-over device by means of which the brake unit can be changed over from a first value of a front-axle/rear-axle brake-force distribution, selected to ensure as high a dynamic stability of the vehicle as possible during a braking operation, to a second value of the front-axle/rear-axle brake-force distribution, at which the rear-axle brake force fraction is higher.
Inasmuch as this is intended to ensure a matching of the brake-force distribution to predetermined load states of the vehicle for the purpose of ensuring the best possible braking effect, a displacement switch which operates as a function of the load can be provided in order that the change-over takes place automatically according to need.
Should the vehicle be equipped with an anti-lock system of conventional design and functioning, criteria and technical measures suitable for the change-over and for carrying it out can be obtained by having the brake unit equipped with an anti-lock system, characterized in that the distribution is changed over to the value of the installed brake-force distribution, corresponding to the higher rear-axle brake-force fraction, when the brake pedal is adjusted and, when the anti-lock control responds, having it changed back to the brake-force distribution value corresponding to a higher dynamic stability. Additionally in the event of the failure of the anti-lock system, the change-over device can change back to the value of the front-axle/rear-axle brake-force distribution corresponding to a higher dynamic stability.
A design of a change-over device of this type which is simple in terms of construction and control is provided by having the housing bore of the housing in which the secondary piston and the annular piston of the primary-piston arrangement are guided displaceably in a pressure-tight manner in a tandem arrangement, possess between a first bore portion, forming the radial limitation, of the secondary outlet-pressure space and a second bore portion forming the limitation, of that part space of the primary outlet-pressure space arranged coaxially relative to the secondary outlet-pressure space, a middle portion, the diameter d of which is smaller than the diameter D of the first bore portion and also smaller than the diameter of the second bore portion receiving the primary-piston arrangement. Here the secondary piston is equipped with two flanges, guided displaceably in a pressure-tight manner in the middle bore position to ensure the pressure-tight delimination of the primary outlet-pressure space relative to the middle bore portion. These flanges define an annular piston flange space extending between them. A brake-force distribution solenoid control valve, in the basic position, shuts off the annular space from the primary outlet-pressure space and relieves pressure towards a brake-fluid reservoir. In an energized position (I), the valve shuts off the brake-fluid reservoir and connects the annular space with the primary outlet-pressure space brake unit. An electronic control unit generates a signal for activating the brake-pressure distribution control valve so as to move it into its energized position I, when and as long as the brake unit is actuated and the anti-lock system of the vehicle has not responded.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.